Shrink tunnel and method of shrinking film on articles

ABSTRACT

This invention is directed to shrink tunnel including an inverted U-shaped housing having a hot air inlet on each side near the bottom thereof and an air outlet below the top of the packages passing through the housing with the inlets and outlets positioned to impinge air against the ends of a package passing through the tunnel and withdraw a large part of the air from the tunnel before it can rise above the top of the package; all as further described herein.

United States Patent Bell [451 June 13, 1972 SHRINK TUNNEL AND METHOD OF SHRINKING FILM ON ARTICLES Re e Ci ed [72] Inventor: Sydney George Frederick Bell, Altona, UNITED STATES PATENTS Austral 3,330,094 7/1967 Ford ..53/1s4 x [73] Assignee: W. R. Grace & Co., New York, NY. 3,402,475 9/1968 Johansen et al ..53/1 84 X [22] Filed: May 1970 Primary Examiner-Theron E. Condon 21 Appl 51 407 Assistant Examiner-Eugene F. Desmond At!0rneyJohn J. Toney, William D. Lee, Jr. and Edward J. Related 1.1.8. Application Data Hanson, Jr.

[62] Division of Ser. No. 682,306, Nov. 13, 1967, Pat. No. [57] ABSTRACT This invention is directed to shrink tunnel including an in- [30] Foreign A plication Priority Data verted U-shaped housing having a hot air inlet on each side near the bottom thereof and an air outlet below the top of the NOV. I8, Australia p g p g h g the housing the inlets and lets positioned to impinge air against the ends of a package [52] US. Cl ..53/30, 34/225, 53/184, passing through the tunnel and withdraw a large part ofthe air 219/388 from the tunnel before it can rise above the top of the [51] Int. Cl ..B65b 53/06 package; all as further described herein [58] Fleld of Search ..53/30, 184; 34/216, 218, 219,

3 Claims, 6 Drawing Figures PATENTEDJUN 13 m2 SHEET 10F g INVENTOR SYDNEY G. F. BELL BY ATTORNEY SHRINK TUNNEL AND METHOD OF SHRINKING FILM ON ARTICLES This application is a division of U. S. application Ser. No. 682,306, filed Nov. l3, 1967 now US. Pat. No. 3,526,752.

This invention relates to packaging and is concerned particularly with the production of packages wherein packaged articles or products are supported within an open-topped con tainer and covered by a sheet of heat shrinkable material, the edges of which overlie the rim of the container and are heat shrunk firmly against the walls of the container.

The invention has particular, but not exclusive, application to the production of packages in which a number of articles, for example, cans filled with a product are supported on a close-fitting tray and covered by a wrap of shrinkable plastic film. Such packs, which for the sake of convenience will hereinafter be referred to as master packs have a number of advantages over conventional cardboard canon packs. They allow goods to be much more easily displayed since it is simply necessary to break open the plastic film and place the tray containing the articles on a display shelf. Goods supplied in cardboard cartons must be removed from the cartons and then stacked on the shelf for display. Master packs also greatly reduce carton disposal problems.

Master packs are generally produced by stacking the articles to be packed in a tray and forming an open-ended tubular sleeve of heat shrinkable plastic film around the tray and the articles. The partially formed pack is then passed through a heating tunnel which heats the film and causes it to shrink. The ends of the open sleeve must be shrunk considerably so that they will lie flat against the ends of the pack and this requires that the ends of the sleeve be shrunk much more than the top, bottom and sides. Therefore, to manufacture such packs satisfactorily, there should be some provision for accurately controlling the amount of shrinkage over the various parts of the film. Conventional heat shrinking tunnels have not proved satisfactory and it is an object of this invention to provide an improved tunnel whereby the shrinkage of the film can be more accurately controlled.

According to the invention there is provided aheating tunnel comprising a top and two sides arranged in a generally inverted-channel-shaped configuration or U-shaped configuration and formed by triple walls defining therebetween an inner air flow duct and an outer air flow duct, air outlet openings for the inner duct distributed over the inner of the triple walls, a port in the intermediate of the triple walls for the passage of air from the inner duct into the outer duct, air inlet means for the outer duct to direct air from the outer duct, inwardly into the bottom part of the tunnel from both sides of the tunnel, air circulating means to draw air through said outlet openings into the inner duct and pass it via the port and the outer duct to said air inlet means, heating means to heat the air passing to the inlet means, and damper means selectively to obstruct and to clear the air outlet openings.

Preferably the air outlet openings are disposed in rows extending longitudinally of the tunnel and the damper means comprises a plurality of damper plates each slidably mounted on said inner wall to cover and uncover the holes of a row by sliding movement thereof.

By an aspect of the invention a method is provided for heat shrinking a heat shrinkable thermoplastic film about an article. The article is first enclosed in the film to form a package. This package is passed through a heat shrink zone. Hot air is introduced into a lower portion of the heat shrink zone and forcibly impinged onto the lower portions of opposing sides of the package. A large portion of the hot air is withdrawn from thezone through at least one outlet positioned below a plane passing through the top of the article and above the lower portion of the zone into which the hot air was introduced.

In order that the invention may be more fully explained, one preferred form of shn'nk tunnel constructed in accordance therewith will now be described in detail with respect to the accompanying drawings in which:

FIG. I is atransverse cross section through the shrink tunnel and a partially formed master pack located therein;

FIG. 2 is a cross section on the line 2-2 in FIG. 1;

FIG. 3 is a partial cross sectional view of the master pack after heat-shrinkage;

FIG. 4 is a partial cross sectional view of part of the tunnel corresponding to the view shown in FIG. 1 but to an enlarged scale and illustrating movement of a control flap for an air inlet slot therein;

FlG. S is a partial perspective view of part of the tunnel; and,

FIG. 6 is a perspective view of the resultant master pack after shrinkage.

FIGS. 1 and 2 show a shrink tunnel denoted generally as 11 in which there is disposed a partially formed master pack denoted generally as 12. Tunnel 11 is approximately 3 feet long and has a 30 inch wide and 15 inch high tunnel opening 13. It is open at the bottom and is triple-walled, being comprised of three inverted channel shaped walls 14, 15, 16 of successively decreasing size located one within the other. The space 17 between the outermost wall 14 and the intermediate wall 15 serves as an air delivery duct for supply of heated air into the tunnel and the space 18 between intermediate wall 15 and innermost wall 16 serves as an air return duct via which air is extracted from the tunnel. The tunnel is fitted with an outer casing 22 lined internally with a layer of heat insulating material 23 and is mounted between support frame members 24a and 24b.

A high speed, high pressure air impeller 19 is mounted in the upper part of air supply duct 17 and is driven via a shaft 20 by an electric motor 21 mounted externally of the tunnel. Impeller 19 rotates about a vertical axis and draws air from return duct 18 through a port 26 made directly below it in the intermediate wall 15. Port 26 is fitted with a throat ring 27 to direct the air into the impeller intake and a number of vertically spaced l0 kw electric heating rods 28 extend in rectangular formations around the impeller.

Impeller 19 rotates at approximately 2,800 revolutions per minute and blows air outwardly over heating rods 28 and then downwardly through the vertical legs of air delivery duct 17. The heated air is directed inwardly into the tunnel from the bottom ends of duct 17 via four slot inlets 29 arranged two to each side of the tunnel and extending longitudinally along it. The air issues from slot inlets 29 in the form of horizontal curtain jets. The lower edge of each slot inlet 29 is defined by a swingable flap 31 which is hinged to the outer tunnel wall 14 at 32 and is connected by a pivot pin 33 to an adjustment screw 34 which passes through an opening in a lug 36 on one of the frame support members 24 and can be moved up and down by rotation of an adjusting nut 37. By rotating the appropriate adjusting nut 37, each flap 31 can be swung to vary the width of its slot inlet 29 from one-half inch down to zero. FIG. 4 shows one slot inlet in a fully opened condition whereas FIG. 1 shows the same slot inlet in a partially closed condition.

The inner wall 16 of tunnel 11 is perforated by holes 38 to allow air to be extracted from the tunnel into return air duct 18. Holes 38 are arranged in eleven longitudinally extending rows which are spaced up the two sides and across the roof portion of the wall. The holes of each row are 1 inch in diameter and are disposed at 2 inch longitudinal spacing. Each row of holes 38 is provided with a damper strip 39 which overlies the holes of the row and is slidable in a pair of channel guides 41 fastened to the inner face of wall 16. Each damper strip 39 is perforated by a row of one inch diameter holes 42 arranged at two inch longitudinal spacing which, by sliding the damper strip, can be moved into and out of register with the holes 38 of the appropriate row. Thus the holes 38 of each row can be selectively obstructed or cleared by adjustment of the appropriate damper strip. Tunnel 11 is installed, so as to straddle part of the upper run of a P.T.F.E. (Polytetrafluoroethylene) coated fiber glass conveyor belt 43.

To form a master pack according to the method of this invention an open ended and continuous tubular sleeve of heat shrinkable plastic film is formed around the article to be packaged with the outer ends of the sleeve projecting beyond the article at both open ends. The sleeve enclosed article is assed into a shrink tunnel with the open ends of the sleeve oriented towards the sides of the tunnel. Hot air curtain jets are directed relatively horizontally against the open ends of the sleeve from the lower region of both sides of the tunnel and below the medial region of the open ends of the sleeve. A large part of the hot air is withdrawn from the tunnel at a height below the top of the article. The horizontal curtain jets of hot air that are directed against the open ends of the sleeve enter the open ends of the sleeve and balloon the ends of the film out from the article. The ballooned end portions of the film are shrunk more rapidly than the other portions of the film. The end portions of the film are thereby shrunk taut and flat about the ends of the article forming a tubular sleeve with smooth constricted ends confining the article therein.

To form a master pack from a number of cans, the cans 44 are placed in a tightly fitting fiber-board tray 46 and an open ended tube 47 of a heat shrinkable plastic film, for example an irradiated polyethylene film, is fonned around the tray and the cans. Tube 47 may be formed by a wrapping machine of known type. The cans 44 and tray 46 are enclosed by tube 47 and then placed on conveyor belt 43 so that tube 47 is extending transversely of the belt. The partially formed pack is then fed by the conveyor belt towards the heating tunnel. Adjacent the tunnel entry, the conveyor passes over a hot plate/cold plate sealer and cooler unit which heat fuses and sets the bottom lap join of the film. The hot plate is electrically heated by 2 kw heaters with its temperature controlled by a thermostat and the cold plate is formed by a stainless steel water tank having internal circulation channels. This portion of the apparatus forms no part of the present invention and is not illustrated in the drawings.

After passing over the hot plate/cold plate sealer and cooler unit, the partially formed package enters the heating tunnel as shown in FIGS. 1 and 2. The horizontal curtain jets of hot air issuing from slot inlet 29 are located just above conveyor belt 43 and provide maximum hot air impact on the open extended ends of the tube 47. These ends are shrunk considerably so that they shrink flat against the ends of the pack as shown in FIG. 3, leaving an opening 48 at each end of the pack surrounded by film which has been greatly thickened at 49 by the extensive shrinking. The curtain jets also act to hold the ends of the film tube ballooned out from the pack during shrinkage, so preventing the film from folding upon itself which could lead to faulty shrinking.

The amount of shrinkage of the sides and top of the film tube 47 can be varied with respect to the shrinkage at the ends by adjusting damper strips 39 and the rate of shrinkage at the ends can be controlled by appropriate adjustment of flaps 31. Thus the volume of hot air directed into the tunnel, and the volume of hot air subsequently passing over the sides and top of the pack can be controlled to give an optimum distribution of shrinkage to produce an adequately tight package without overstressing the film. The air outlet openings 38 and the air inlet means 29 are positioned relative to one another to not only impinge air against the ends of a package passing through the tunnel but to withdraw at least a large part of the air from the tunnel so directly after its injection into the tunnel that the air cannot rise above the-top of the package. A substantial portion of the air outlet openings 38 are below the height of the packages the U-shaped housing will accomodate. The resultant master pack is shown in FIG. 6.

Master packs which have been produced by the illustrated apparatus have proved to be very satisfactory. Because of the thickening of the film around openings 48, these openings may be used as finger holes for lifting purposes. The openings also allow the circulation of air between containers 44 to prevent condensation which could damage metal containers. However, it is to be understood that the above described apparatus has been given by way of example only and that the invention is not in any way limited thereto but includes all modifications and adaptations which fall within the spirit and scope of the appended claims.

We claim 1. A method for heat shrinking a heat shrinkable thermoplastic film about an article comprising forming said film into an open ended and continuous tubular sleeve around the article with the outer ends of the sleeve projecting beyond the article at both open ends to form a package, passing said package through a heat shrink zone with the open ends of the sleeve oriented towards the sides of the zone, directing a relatively horizontal flow of hot air from the bottom part of the sides of the heat shrink zone and from below the medial region of the package against the lower portions of the opposing sides of the package having the open ends of the sleeve, withdrawing a large portion of said hot air from said zone at a height below the top of the article and above said lower portion of said zone, shrinking the projecting open ends of the sleeve more rapidly than the other portions of the film and taut about the ends of the article.

2. The method of claim 1 wherein the relatively horizontal flow of hot air is in the form of jets of hot air which enter the open ends of the sleeve and balloon the ends of the film out from the article and these ballooned portions of the film are the ends that are shrunk more rapidly than the other portions of the film and substantially flat against the ends of the article and said hot air is withdrawn through at least one outlet positioned below the top of the article and above said lower portion of said zone.

3. The method of claim 2 wherein the jets of hot air are curtain jets. 

1. A method for heat shrinking a heat shrinkable thermoplastic film about an article comprising forming said film into an open ended and continuous tubular sleeve around the article with the outer ends of the sleeve projecting beyond the article at both open ends to form a package, passing said package through a heat shrink zone with the open ends of the sleeve oriented towards the sides of the zone, directing a relatively horizontal flow of hot air from the bottom part of the sides of the heat shrink zone and from below the medial region of the package against the lower portions of the opposing sides of the package having the open ends of the sleeve, withdrawing a large portion of said hot air from said zone at a height below the top of the article and above said lower portion of said zone, shrinking the projecting open ends of the sleeve more rapidly than the other portions of the film and taut about the ends of the article.
 2. The method of claim 1 wherein the relatively horizontal flow of hot air is in the form of jets of hot air which enter the open ends of the sleeve and balloon the ends of the film out from the article and these ballooned portions of the film are the ends that are shrunk more rapidly than the other portions of the film and substantially flat against the ends of the article and said hot air is withdrawn through at least one outlet positioned below the top of the article and above said lower portion of said zone.
 3. The method of claim 2 wherein the jets of hot air are curtain jets. 